Studies will be carried out to determine the mechanism(s) whereby hormones regulate the metabolism of the mammary gland. Accordingly, it will be determined if prolactin regulates the levels of cyclic AMP or cyclic GMP in primary epithelial cell cultures of the mammary gland. The possible effects of hormones on prostaglandin biosynthesis will also be determined by measuring prostaglandin levels in hormone-treated mammary gland explants and in the primary epithelial cell cultures. Further experiments will be carried out to determine if the activation of a membrane associated phospholipase A may be one of the primary mechanisms whereby prolactin acts on the mammary gland. The effect of prolactin on phospholipase A activity in microsomal fractions and in plasma membrane fractions of the mammary gland will be determined.